Letter pubs.acs.org/OrgLett
Oxidative Dimerization of 2‑Oxindoles Promoted by KOtBu‑I2: Total Synthesis of (±)-Folicanthine Santanu Ghosh, Saikat Chaudhuri, and Alakesh Bisai* Department of Chemistry, Indian Institute of Science Education and Research, Bhopal - 462 066, MP, India S Supporting Information *
ABSTRACT: A ‘transition-metal-f ree’ oxidative coupling of 2-oxindoles has been demonstrated in the presence of 1.2 equiv each of potassium tert-butoxide and iodine. The method yields a diverse range of structurally different homo- and heterodimerized 2oxindoles bearing vicinal all-carbon quaternary centers of great synthetic importance. A radical-driven pathway has been tentatively proposed. C2-Symmetric1 dimeric cyclotryptamine alkaloids (1a−c, Figure 1) with a labile C3a−C3a′ σ-bond possess a diverse array of
palladation of indole (Scheme 1). We envisioned that folicanthine 1b and related cyclotryptamine alkaloids can be Scheme 1. Pd-Catalyzed Oxidative Dimerization
synthesized from a common intermediate (±)-2 (Figure 1), which in turn can be accessed from a direct oxidative coupling of 2-oxindole (±)-4 (Figure 1). Herein, we report an expeditious entry to dimeric 2-oxindoles bearing a variety of esters (8 and 9) and alkyl functionality (2) via a direct oxidative coupling of 3substituted 2-oxindoles (6, 7, and 4) promoted by KOtBu-I2, under mild conditions. Initially, we thought to synthesize the dimeric 2-oxindoles10 possessing bisesters functionalities (8 and 9) via a Pd-catalyzed oxidative coupling11 of 2-oxindole derivatives (6 and 7) (Scheme 1). We envisioned that, in the presence of a suitable base, (±)-6a would generate enolate 10 (Scheme 2), which upon subsequent ligand exchange with Pd(OAc)2 could lead to the formation of Pd(II) intermediate 11c via the intermediacy of 11a and 11b. Ultimately, the direct coupling of two C−H bonds of 2-oxindoles (±)-6a could be achieved following a reductive elimination pathway. The process can be catalytic in the presence of a
Figure 1. Selected bis-indole alkaloids (1a−c) with C2-symmetry.
biological activities.2 It is the presence of two vicinal all-carbon quaternary stereocenters in these alkaloids which are a challenge to the synthetic community, and hence they are attractive synthetic targets.3 One of the plausible biogenetic pathways to achieve this involves a direct C−C bond formation via oxidative coupling4,5 of two C−H bonds. Synthetic approaches in this direction are mainly concentrated toward the dimerization of 2oxindoles such as 3a6a,b and 3b6c (Figure 1). However, these methods generally afford dimeric 2-oxindole products in poor to moderate yields, which might be due to the associated difficulty in the formation of a C−C bond containing a vicinal all-carbon quaternary center. Other approaches to dimeric cyclotryptamine alkaloids include direct oxidative dimerization of 3-substituted indoles.7a−c,8 In 2010, Shi et al. demonstrated an oxidative homodimerization of indole derivatives toward 3,3′-linked bis-indole scaffolds (5) via Pd-catalysis,9 where authors proposed formation of C-3 © 2015 American Chemical Society
Received: January 5, 2015 Published: March 6, 2015 1373
DOI: 10.1021/acs.orglett.5b00032 Org. Lett. 2015, 17, 1373−1376
Letter
Organic Letters
efficient to afford 8a in 66−68% yields with dr 2.0:1 (entries 6 and 7). Also, tBuOK was found to be superior over other bases such as Cs2CO3 and tBuONa used in this reaction (entries 8−9). Surprisingly, the same reaction, when performed in the absence of Pd(II) (entry 10, Table 1), afforded 8a in 84% yield with dr 1.9:1. A brief solvent screening showed that nonpolar solvent such as toluene (entry 11) was inferior to polar aprotic solvents such as dimethyl sulfoxide (DMSO) and N,Ndimethylformamide (DMF) (entries 12−13), yielding 8a in 74% and 81% isolated yields, respectively, with almost similar dr. Optimization using different bases such as tBuONa, Na2CO3, K2CO3, and Cs2CO3 revealed that the product 8a can be obtained in the range of 62−75% isolated yields with up to 2.4:1 dr (entries 14−17).13 These results prompted us to revise our hypothesis of the Pd(II)-catalyzed sequence shown in Scheme 2. Since, palladium does not play a significant role in the reaction of 6a to dimeric 8a, we proposed a more reasonable alternative route involving a radical mediated intermolecular oxidative coupling. A single electron transfer (SET) from enolate 10 to an oxidant leads to the formation of radical intermediate 12a (Scheme 3).14 The latter has the resonating structure, a 3° radical
Scheme 2. Our Initial Hypothesis
sacrificial oxidant, which would help bring back the active catalyst Pd(II). Preliminary experiments were performed with N-methyl-2oxindole-3-methylcarboxylate (6a) in the presence of 1.2 equiv of tBuOK as base and 1.2 equiv of various oxidants using 10 mol % Pd(OAc)2 as the catalyst in DMF at 0 °C−rt. Several oxidants such as FeCl3, Cu(OAc)2, iodosobenzene diacetate (PIDA), bistrifluoroacetate iodoxybenzene (PIFA), and I2 were screened in the coupling reaction. It was found that molecular iodine12 was the best oxidant to afford (±)-and meso-8a in 61% isolated yield (dr 1.9:1) in 20 min (Table 1, entries 1−5). Switching to THF as solvent, the yield was further increased to 68% (dr 1.9:1) (entry 6). We observed that Pd(OAc)2 and Pd(TFA)2 were equally
Scheme 3. Revised Proposed Pathway of Dimerization
Table 1. Optimization of Oxidative Coupling
sl. no.
solvent
base
oxidants
time
yielda (%)
drb
c
DMF DMF DMF DMF DMF THF THF THF THF THF PhMe DMSO DMF THF THF THF THF
KOtBu KOtBu KOtBu KOtBu KOtBu KOtBu KOtBu Cs2CO3 NaOtBu KOtBu KOtBu KOtBu KOtBu NaOtBu Na2CO3 K2CO3 Cs2CO3
FeCl3 Cu(II)e PIDA PIFA I2 I2 I2 I2 I2 I2 I2 I2 I2 I2 I2 I2 I2
1h 1h 30 min 30 min 20 min 20 min 20 min 40 min 40 min 15 min 30 min 15 min 15 min 15 min 15 min 15 min 15 min
− trace 43f 38f 61 68 66 51f 46f 84 31f 74 81 62 68 75 72
− − 1.4:1 1.3:1 1.9:1 1.9:1 2.0:1 2.0:1 1.2:1 1.9:1 1.3:1 1.7:1 1.9:1 1.1:1 1.5:1 2.4:1 1.9:1
1 2c 3c 4c 5c 6c 7d 8c 9c 10 11 12 13 14 15 16 17
(12b) at the pseudobenzylic position, which simply dimerizes to afford expected product 8a (Scheme 3). In fact, a reaction between tBuOK and iodine is known to produce tBuOI,15 which is reported to be a good radical initiator in a variety of organic transformations.16,17 In addition to iodine, simple oxidants such as N-halo succinimides such as NIS18 and NBS also afforded product 8a in 83% and 69% yields, respectively, with dr 1.6:1 (Table 2, entries 1−2) and we did not observe any traces of halides 13a−b. Interestingly, well-known oxidizing agents, such as PIDA and PIFA,19 which follow a radical mechanism, also afforded dimeric product 8a in 45−52% yields (entries 3−4). Thus, based on this Table 2. Optimization of Intramolecular-Dehydrogenative Coupling
a
Reactions were carried out on a 0.25 mmol of 6a in the presence of 0.30 mmol of base and oxidants in 1 mL of solvent at 0−25 °C, and isolated yields of 8a are reported. bDr determined from 1H NMR of unpurified reaction mixture. c10 mol % Pd(OAc)2 was used as catalyst. d 10 mol % Pd (TFA)2 was used as catalyst. eCu(OAc)2·H2O was used as oxidant. fMixture of products for rest of the mass balance.
entry
oxidants
time
yielda (%)
drb
1 2 3 4
NIS NBS PIDA PIFA
15 min 15 min 25 min 25 min
83 69 52c 45c
1.5:1 1.6:1 1.7:1 1.4:1
a
Isolated yields of 8a. bDr determined from 1H NMR of unpurified reaction mixture. cMixture of products for rest of the mass balance. 1374
DOI: 10.1021/acs.orglett.5b00032 Org. Lett. 2015, 17, 1373−1376
Letter
Organic Letters evidence, it is quite reasonable to understand that our methodology follows a radical mediated process. With the optimized conditions, we then explored the substrate scope using a variety of N-alkyl-2-oxindole-3-carboxylates (6, see Supporting Information for synthetic procedure) to afford dimeric 2-oxindoles. As evidenced from Figure 2, a variety of
Figure 3. Substrate scope with various allylic type esters.
Scheme 4. Scope of Heterodimerization
Figure 2. Substrate scope of oxidative dimerization.
dimeric 2-oxindoles sharing vicinal all-carbon quaternary centers (8a−r) were obtained in 62−84% yields with a dr up to 3.9:1. The dr of the products can be further improved to >20:1 following recrystallization in a few cases to afford active isomer (±)-8.20 To further extend the scope, a variety of N-methyl and benzyl substituted 2-oxindole-3-carboxylates (7, see Supporting Information for detailed method) having allyl, methallyl, prenyl, and geranyl esters with a diversely substituted 2-oxindole core were subjected to standard coupling conditions, and the results are shown in Figure 3. Rewardingly, a variety of dimeric 2-oxindoles 9a−l were prepared in good yields with a dr up to 2.8:1. Importantly, these dimeric 2-oxindoles are potential substrates that can provide access to enantioenriched bis-2-oxindoles with vicinal all-carbon quaternary stereocenters21,22 under the influence of a catalytic nonracemic Pd-complex.23 Later, a search for a more generalized dimerization protocol prompted us to carry out heterodimerization of two structurally different N-alkyl-2-oxindole-3-carboxylates (Scheme 4). Consequently, we observed that an efficient heterodimerization could also be realized under optimized conditions (see Supporting Information for details). When two different ester substrates were subjected to 1.2 equiv of each KOtBu and I2, heterodimerized products 9m−n were obtained in 50−52% yield with a moderate dr of 2.4:1. In addition, we also isolated homodimerized products in these processes in ∼10− 13% yields (Scheme 4). We have also seen that an isolated yield of the heterodimerization product is dependent on the concentration of the reaction medium. With the dilution, an isolated yield of 9m also increased (see Supporting Information for details). These heterodimerized skeletons represent an
efficient platform for the synthesis of unsymmetrical dimeric indole alkaloids. Apart from this, different N-protected substrates, such as 3methyl-N-Boc-2-oxindole 15a, also undergo dimerization without an event to afford 14a in 73% with an ∼1.1:1 dr (Figure 4) thus increasing the scope of the method even further. However, the reaction faced limitations with substrate 16a, leading to 45− 50% recovery of starting material along with decomposition of the rest of the mass balance (Figure 4). In the case of 2-oxindole 17, the starting material was consumed but no expected product was isolated and always a multitude spots on TLC were observed. These results also suggested a tertiary radical is probably responsible for the oxidative coupling. Notably, Nb-phth protected N-methyl-2-oxindole (±)-4 afforded dimeric 2 in 56% yield with ∼1.2:1 dr, which on subsequent deprotection of the phthalimido group followed by carbamate protection afforded 19 in 70% yield with ∼1.2:1 dr. The major trans isomer was separated in column chromatography followed by Red-Al treatment affording (±)-folicanthine 1b in 68% yield (see Supporting Information for details). In summary, we have developed a novel and efficient oxidative coupling protocol which allows the synthesis of a structurally diverse range of dimeric 2-oxindoles with vicinal quaternary centers. The reaction is mediated by KOtBu and I2 under mild conditions, and additional research will focus on the develop1375
DOI: 10.1021/acs.orglett.5b00032 Org. Lett. 2015, 17, 1373−1376
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Organic Letters
(6) (a) Hino, T.; Yamada, S.-i. Tetrahedron Lett. 1963, 4, 1757. (b) Hendrickson, J. B.; Goschke, R.; Ress, R. Tetrahedron 1964, 20, 565. (c) Fang, C.-L.; Horne, S.; Taylor, N.; Rodrigo, R. J. Am. Chem. Soc. 1994, 116, 9480. (7) (a) Scott, A. L.; Guo, L.; McCapra, F.; Hall, E. S. J. Am. Chem. Soc. 1964, 86, 302. (b) Ishikawa, H.; Takayama, H.; Aimi, N. Tetrahedron Lett. 2002, 43, 5637. (c) Li, Y.-X.; Wang, H.-X.; Ali, S.; Xia, X.-F.; Liang, Y.-M. Chem. Commun. 2012, 2343. (8) (a) Tadano, S.; Mukaeda, Y.; Ishikawa, H. Angew. Chem., Int. Ed. 2013, 52, 7990 and references cited. For radical mediated dimerization, see: (b) Movassaghi, M.; Schmidt, M. A. Angew. Chem., Int. Ed. 2007, 46, 3725. (c) Kim, J.; Ashehurst, J. A.; Movassaghi, M. Science 2009, 324, 238. (d) Kim, J.; Movassaghi, M. J. Am. Chem. Soc. 2010, 132, 14376. (9) Li, Y.; Wang, W.-H.; Yang, S.-D.; Li, B.-J.; Feng, C.; Shi, Z. J. Chem. Commun. 2010, 46, 4553. (10) Our one-pot synthesis of dimeric 2-oxindoles afforded product 3 only in 30−42% yields; see: Ghosh, S.; De, S.; Kakde, B. N.; Bhunia, S.; Adhikary, A.; Bisai, A. Org. Lett. 2012, 14, 5864. (11) For direct oxidative coupling, see: (a) DeMartino, M. P.; Chen, K.; Baran, P. S. J. Am. Chem. Soc. 2008, 130, 11546 and references therein. (b) Jia, Y. X.; Kündig, E. P. Angew. Chem., Int. Ed. 2009, 48, 1636. (c) Perry, A.; Taylor, R. J. K. Chem. Commun. 2009, 3249. (d) Guo, F.; Konkol, L. C.; Thomson, R. J. J. Am. Chem. Soc. 2011, 133, 18. (e) Bhunia, S.; Ghosh, S.; Dey, D.; Bisai, A. Org. Lett. 2013, 15, 2426. (12) For iodine in homodimerization reactions, see: (a) Alvarez-Ibarra, C.; Csákÿ, A. G.; Colmenero, B.; Quiroga, M. L. J. Org. Chem. 1997, 62, 2478. (b) Kuo, W.-J.; Chen, Y.-H.; Jeng, R.-J.; Chan, L.-H.; Lin, W.-P.; Yang, Z.-M. Tetrahedron 2007, 63, 7086. (13) We sincerely thank one of the reviewers for investigating the effect of simple bases, such as tBuONa, Na2CO3, K2CO3, and Cs2CO3, under the optimized conditions. (14) The vinyloxy radical has been proposed earlier by Baran and coworkers: Richter, J. M.; Whitefield, B. W.; Maimone, T. J.; Lin, D. W.; Castroveijo, P. M.; Baran, P. S. J. Am. Chem. Soc. 2007, 129, 12857. (15) (a) Montoro, R.; Wirth, T. Org. Lett. 2003, 5, 4729. (b) Akhtar, M.; Barton, D. H. R. J. Am. Chem. Soc. 1964, 86, 1528. (16) We sincerely thank the reviewers for their valuable suggestions to validate the reactions using a preformed well-known radical generator t BuOI. (17) We have also conducted the oxidative dimerization in the presence of in situ generated tBuOI (see Supporting Information for details) and found that oxidative dimerized products can be obtained in synthetically useful yields. (18) For use of NIS as an oxidant in an oxidative bond-forming reaction, see: (a) West, S. P.; Bisai, A.; Lim, A. D.; Narayan, R. R.; Sarpong, R. J. Am. Chem. Soc. 2009, 131, 11187. (b) Foo, K.; Newhouse, T.; Mori, I.; Takayama, H.; Bran, P. S. Angew. Chem., Int. Ed. 2011, 50, 2716. (19) For a recent example, see: Wang, J.; Yuan, Y.; Xiong, R.; ZhangNegrerie, D.; Du, Y.; Zhao, K. Org. Lett. 2012, 14, 2210. (20) The major diastereomer of our oxidative process is the active one, which was confirmed from HPLC analysis of one of the products (9a). (21) The dimeric 2-oxindoles of the type 9a can be converted to C2symmetric 2-oxindoles with vicinal all-carbon quaternary stereocenters. See our report: Ghosh, S.; Bhunia, S.; Kakde, B. N.; De, S.; Bisai, A. Chem. Commun. 2014, 50, 2434. (22) For pioneering work on enantioselective double decarboxylative allylations, see: (a) Trost, B. M.; Osipov, M. Angew. Chem., Int. Ed. 2013, 52, 9176. (b) Enquist, J. A., Jr.; Stoltz, B. M. Nature 2008, 453, 1228. (23) For Pd-catalyzed asymmetric prenylations and geranylations, see: Trost, B. M.; Malhotra, S.; Chan, W. H. J. Am. Chem. Soc. 2011, 133, 7328.
Figure 4. Further scope.
ment of catalytic versions of this method as well. The high synthetic value of oxidative coupling products ensures further improvement in this field and its applications in the total synthesis of dimeric cyclotryptamine alkaloids.
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ASSOCIATED CONTENT
S Supporting Information *
General experimental procedures and analytical data for all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support from the DST (SR/S1/OC-54/2011) and the CSIR [02(0013)/11/EMR-II], Govt. of India is gratefully acknowledged. S.G. and S.C. thank the CSIR and UGC, New Delhi, for the SRF and JRF, respectively.
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REFERENCES
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